On my survival world, I have a zombie spawner that I found and set up to be a zombie villager farm. I had finally trapped a zombie villager and I went to go get supplies to cure it, but when I got back the boat i put him in was empty along with all the other mobs in the area.
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This is not a duplicate of MCPE-21416. It is just the despawning rules working as intended. See MCPE-66928.
This issue has already been reported. Duplicate MCPE-21416.
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